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Crystal structures of the C-terminally truncated endoglucanase Cel9Q from Clostridium thermocellum complexed with cellodextrins and Tris.
Chembiochem : a European Journal of Chemical Biology 2019 January 5
Endoglucanase CtCel9Q is one of the enzyme components of the cellulosome, which is an active cellulase system in the thermophile Clostridium thermocellum. The precursor form of CtCel9Q comprises a signal peptide, a glycoside hydrolase family 9 (GH9) catalytic domain, a type 3c carbohydrate binding module (CBM), and a type I dockerin domain. Here, we report the crystal structures of the C-terminally truncated CtCel9Q (CtCel9QΔc) complexed with Tris, Tris + cellobiose, cellobiose + cellotriose, cellotriose, and cellotetraose at resolutions of 1.50, 1.70, 2.05, 2.05 and 1.75 Å, respectively. CtCel9QΔc forms a V-shaped homodimer via residues Lys529-Glu542 on the type 3c CBM, which pairs two β-strands (β4 and β5 of the CBM) together. In addition, a disulfide-bond was formed between the two Cys535 residues of the protein monomers in the asymmetric unit. The structures allow the identification of four minus (-) subsites and two plus (+) subsites, which is important for further understanding the structural basis of cellulose binding and hydrolysis. Moreover, in the oligosaccharide-free and cellobiose-bound CtCel9QΔc structures, a Tris molecule was found to be bound to three catalytic residues of CtCel9Q and occupied subsite -1 of the CtCel9Q active-site cleft. Furthermore, the enzyme activity assay in the presence of 100 mM Tris showed that the CtCel9Q hydrolase activity was almost completely suppressed by Tris.
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